the egersund dykes (sw norway): a robust early gji ... · geophys. j. int. (2007) 168, 935–948...

Geophys. J. Int. (2007) 168, 935–948 doi: 10.1111/j.1365-246X.2006.03265.x

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The Egersund dykes (SW Norway): a robust EarlyEdiacaran (Vendian) palaeomagnetic pole from Baltica

Harald J. Walderhaug,1 Trond H. Torsvik2,3,4 and Erik Halvorsen5

1Department of Earth Science, University of Bergen, Allegt. 41, N-5007 Bergen, Norway. E-mail: [email protected] for Geodynamics, Geological Survey of Norway, Leif Eirikssons vei 39, N-7491 Trondheim, Norway3Institute for Petroleum Technology and Applied Geophysics, Norwegian University of Science & Technology, N-7491 NTNU, Norway4School of Geosciences, Private Bag 3, University of the Witwatersrand, WITS 2050, South Africa5Telemark University College, Lærerskoleveien 40, N-3679 Notodden, Norway

Accepted 2006 October 13. Received 2006 October 13; in original form 2006 January 7

S U M M A R YPalaeomagnetic data for Baltica in the Late Precambrian are highly ambiguous, and havetherefore, given rise to different interpretations concerning the need to explain Varangerianglaciations through snowball Earth conditions. We present new palaeomagnetic data fromthe 616 ± 3 Ma (U-Pb) Egersund dykes in SW Norway, which yield a palaeolatitude of53◦ +16◦/−13◦ for the studied location. This would indicate that the Baltica plate spannedlatitudes between 50◦ and 75◦ S in the Early Ediacaran. The pole position (31◦N, 44◦E,dp/dm = 15/17) confirms earlier studies, but the primary nature of the remanence is nowsupported by two positive contact tests. A new 40Ar/39Ar biotite age of 600 ± 10 Ma fromone of the dykes suggests that remanence acquisition in the dykes took place between 600 and616 Ma. The Egersund palaeomagnetic data demonstrate that Baltica was located at relativelyhigh latitudes at the time of the Varangerian glaciations.

Additional palaeomagnetic sites in the Rogaland Igneous Complex yield a pole position(46◦S, 238◦E, dp/dm = 17/19) that confirms previous studies. The age of this remanencehas traditionally been quoted as c. 930 Ma based on U-Pb ages from the complex. However,previous 40Ar/39Ar hornblende ages of around 870 Ma are now supported by a new 40Ar/39Arbiotite age of 869 ± 14 Ma obtained from a noritic dyke, and we argue that this represents anuplift/cooling age which better represents the age of the remanence in SW Norway.

Key words: Baltica, Ediacaran, Egersund dykes, geochronology, neoproterozoic glaciations,palaeomagnetism.

I N T RO D U C T I O N

The Late Precambrian was a pivotal time in Earth history, with the

breakup of the supercontinent Rodinia and possible ‘snowball Earth’

conditions (Kirschvink 1992) setting the scene for the Cambrian

animal diversification. In this context, reliable palaeomagnetic data

are crucial to determining whether the numerous Late Precambrian

glacial deposits recorded on different continents represent glacia-

tions which were truly global in nature, or may, at least in some

cases, simply be explained by the (latitudinal) positions of individ-

ual continents.

The termination of the Precambrian has conventionally been

termed the Vendian Period (ca. 650–543 Ma), but recently the lat-

est Precambrian has been named Ediacaran (Knoll et al. 2004).

The lower boundary (in Australia) is defined as the contact between

Marinoan glacial rocks and overlying Ediacaran cap carbonates, thus

defining the boundary between a global ice age ending at around

635 Ma (Hoffman et al. 2004; Condon et al. 2005) and the diversi-

fication of soft-bodied life. Two further epochs of global glaciation

are also believed to have occurred at around 730 Ma. (‘Sturtian’) and

580 Ma (‘Gaskiers’) respectively, although the exact dates remain

somewhat uncertain (Halverson et al. 2005).

Specifically for Baltica, a key question is whether the poorly dated

Varangerian tillites (traditionally linked to either the Marinoan or

the Gaskiers glaciations; see Bingen et al. 2005) were deposited at

low or high latitudes. Unfortunately, the existing palaeomagnetic

data for Baltica are ambiguous. In their widely cited data compi-

lation, Torsvik et al. (1996) argued for a high latitude position of

Baltica during the Vendian, based on palaeomagnetic data from Fen

(Oslo region), Sredny (North Russia) and Egersund (SW Norway),

but they pointed out that there is a general lack of reliable palaeo-

magnetic data for Baltica prior to the Early Ordovician. In contrast,

recent palaeomagnetic contributions from the 608 ± 1 Ma (U-Pb)

Sarek dyke swarm in Northern Sweden (Eneroth 2002; Eneroth &

Svenningsen 2004) suggested an equatorial palaeolatitude. This re-

sult has subsequently been referred to as a ‘robust palaeolatitude

C© 2007 The Authors 935Journal compilation C© 2006 RAS

936 H. J. Walderhaug, T. H. Torsvik and E. Halvorsen

Figure 1. Map giving the location of the Egersund dykes within the Sweconorwegian province of SW Norway (top left), regional setting (top right), and

location of the individual dykes within the RIC and Sveconorwegian gneiss complex (bottom) following Antun (1955). Individual palaeomagnetic sampling

sites in the dykes are indicated by open circles, with three additional sampling sites in norites from of the RIC indicated by solid circles.

for Baltica’ by Macouin et al. (2004, p. 396) and used as evidence

for a global Neoproterozoic glaciation event spanning low to high

latitudes. However, the palaeomagnetic data and interpretations re-

ported in Eneroth (2002) and Eneroth & Svenningsen (2004) are

flawed due to a mistake in the measurement orientation convention,

and the authors have subsequently withdrawn both papers (Eneroth

2006a,b). Thus, the Sarek palaeomagnetic results should not be used

to establish the position of Baltica during the Neoproterozoic.

The most widely used palaeomagnetic results which seem to argue

for an intermediate to high latitude position for Baltica at this time

(Storetvedt 1966; Poorter 1972), originate from the now well dated

616 ± 3 Egersund dyke system (Bingen et al. 1998) in southwestern

Norway. However, the existing palaeomagnetic data have been called

into question (e.g. Eneroth & Svenningsen 2004) due to the limited

amount of data, seemingly unstable remanence with a direction close

to the present day field, and the absence of field tests to constrain the

age and stability of magnetization. The aim of the present study is to

resolve the latitudinal ambiguity in the Vendian data for Baltica by

reexamining the palaeomagnetic signature of the Egersund dykes

and the host rocks of the surrounding Rogaland Igneous Complex

C© 2007 The Authors, GJI, 168, 935–948

Journal compilation C© 2006 RAS

The Egersund dykes 937

Table 1. 40Ar/39Ar biotite furnace step heating of samples e-12d (Noritic dyke) and E16D (Egersund dyke)

(weight = 5 mg).

Sample = e-12dJ = .0060870Temp 40Ar/39Ar 38Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39Ar 39Ar 40Ar*a 40Ar*/39K Age (Ma) ±σ

(per cent) (per cent)

500 84.172 0.024 0 105.225 1.787 0.63 63 53.06 505.18 7.24

540 113.395 0.024 0 70.225 1.865 1.29 81.7 92.62 807.06 5.71

600 120.108 0.027 0 92.993 5.183 3.12 77.1 92.61 806.95 2.93

60 140.319 0.022 0 131.373 15.985 8.76 72.3 101.48 868.23 1.88

690 102.872 0.011 0 3.498 44.31 24.4 99 101.82 870.54 0.65

730 102.813 0.012 0 5.047 76.083 51.26 98.5 101.3 867.02 0.48

750 102.559 0.013 0 4.82 49.789 68.83 98.6 101.11 865.76 0.75

770 110.706 0.013 0 28.546 26.939 78.34 92.4 102.25 873.47 0.93

790 120.77 0.021 0.283 59.181 10.899 82.19 85.6 103.32 880.72 3.1

820 121.204 0.015 0 64.62 4.782 83.88 84.2 102.09 872.37 6.29

850 115.086 0.016 0 48.888 3.395 85.08 87.4 100.62 862.39 5.09

890 107.569 0.019 0 31.212 4.934 86.82 91.4 98.32 846.69 3.8

940 102.518 0.017 0 18.507 10.699 90.6 94.6 97.03 837.75 1.43

990 105.72 0.015 0 13.634 10.195 94.2 96.2 101.67 869.54 2.82

1040 110.768 0.01 0 23.84 11.326 98.19 93.6 103.7 883.27 1.93

1090 107.839 0.015 0 14.849 5.118 100 95.9 103.43 881.44 3.35

Sample = E16DJ = .0058540

550 175.715 0.242 0 301.119 0.418 0.65 49.3 86.7 740.96 46.01

600 253.791 0.228 0 593.105 0.676 1.69 30.9 78.5 682.51 23.66

650 635.971 0.2 0.08 1755.251 1.129 3.44 18.4 117.28 943.49 89.66

700 106.862 0.124 0.198 92.714 1.823 6.26 74.4 79.47 689.53 6.17

735 93.873 0.133 0.126 56.478 2.39 9.96 82.2 77.18 672.91 5.72

770 169.774 0.102 0.145 305.084 3.173 14.88 46.9 79.62 690.59 13.08

810 131.515 0.081 0.203 162.435 3.361 20.08 63.5 83.52 718.52 4.35

850 76.108 0.06 0.078 11.981 3.839 26.02 95.3 72.55 638.93 3.65

890 77.201 0.104 0.191 24.507 4.407 32.84 90.6 69.96 619.62 3.2

940 77.438 0.105 0.099 18.156 6.114 42.31 93.1 72.06 635.28 2.82

1000 71.88 0.052 0.04 13.266 10.177 58.06 94.5 67.94 604.38 0.89

1050 72.758 0.059 0.058 13.136 9.668 73.03 94.6 68.86 611.32 2.18

1100 68.685 0.079 0.113 6.045 9.098 87.11 97.4 66.89 596.43 0.94

1150 71.562 0.066 0.217 15.375 8.325 100 93.7 67.02 597.47 0.93

Individual temperature steps are listed with 1σ errors (excluding J ). Uncertainty in J-values for monitors ranged

from 0.1 to 0.8 per cent without including error in monitor age; we incorporated a conservative 1 per cent error in

J-value for all unknowns.a 40Ar* = radiogenic 40Ar.

(RIC), with emphasis on field tests and a more extensive sampling

scheme. In addition we present the first 40Ar/39Ar age for one of the

dykes as well as a noritic dyke from the RIC.

R E G I O N A L S E T T I N G

The Egersund dyke system is located within the The Rogaland and

Vest-Agder Gneiss Complex (RVGC) in SW Norway (Fig. 1). This

complex was highly deformed during the Sveconorwegian orogeny

and subsequently intruded by massif-type anorthosites, the RIC,

between 932 ± 3 and 929 ± 2 Ma (U-Pb zircon ages; Scharer

et al. 1996). RVGC augen gneisses yield U-Pb titanite cooling ages

between 917 ± 2 and 920 ± 4 Ma, whilst 40Ar/39Ar hornblende

yields ages of 875 ± 20 and 867 ± 2 Ma (Bingen & van Breemen

1998; Bingen et al. 1998). The latter probably reflect cooling through

500 ± 25◦C (Harrison 1981).

Two major dyke systems are recognized in SW Norway. The

NE-SW trending Hunnedalen dykes intrude the RVGC and are dated

to 848 ± 27 Ma ( 40Ar/39Ar biotite; Walderhaug et al. 1999). The

second dyke swarm, the WNW-ESE trending Egersund dyke system

(olivine dolerites, dolerites and trachydolerites), mainly intrudes the

RIC but also the RVGC (site 5 and 16 in Fig. 1). The Egersund dykes

are typically vertical, can be traced for tens of kilometres, and vary

in width between 0.3 and 30 m. They often show glassy chilled mar-

gins and skeletal crystals suggesting rapid cooling. A precise U-Pb

baddeleyite age of 616 ± 3 Ma has been reported by Bingen et al.(1998).

E X P E R I M E N TA L DATA

Thirteen sites in the Egersund dykes were examined for palaeomag-

netic analysis (Fig. 1), including contacts and the host rock at three

sites [site 5 (migmatitic gneisses from RVGC) and sites 9 and 16

(Anorthosite from the RIC)]. One dyke was selected for 40Ar/39Ar

geochronology (site 10, Fig. 1). In addition, three noritic dykes be-

longing to RIC were palaeomagnetically tested, and one of these

dykes was also dated by 40Ar/39Ar.

40Ar/39Ar data

The 40Ar/39Ar incremental heating experiments on biotites were

performed at the Geological Survey of Norway argon laboratory.

C© 2007 The Authors, GJI, 168, 935–948



Figure 2. 40Ar/39Ar biotite spectrum for a norite (top) and Egersund dyke sample (Table 1) that shows apparent age as a function of the cumulative fraction of39Ar released. Height of boxes indicates analytical error (±1σ ) about each step. We cite uncertainties at the 2σ level and include uncertainties in J -value. Inset

diagram: inverse isochron for the norite sample after removing the three first low temperature steps and temperature steps 890 and 940◦C (40Ar/36Ar intercept

= 303.5 ± 8). See text for age discussion.

Gas from irradiated samples was released in a step-wise fashion

from a resistance furnace, and the purified gas was analyzed on a

MAP 215–50 mass spectrometer with general analytical protocol

and irradiation parameters similar to Eide et al. (2002). The ages

we cite in the text (either final plateau or isochron ages) are at 2σ

and including error in J -value.

A noritic dyke (Site 6—Sample e-12D in Table 1) yielded

a 40Ar/39Ar biotite weighted mean near-plateau age (WMPA—

weighted on both gas length and individual errors at each tem-

perature step) of 869 ± 14 Ma for steps 8–11 (82 per cent gas;

Fig. 2a). An inverse isochron (removing the three first low temper-

ature steps) yields an identical age (867 ± 14 Ma) but with high

MSWD (31)(40Ar/36Ar intercept = 303.1 ± 14). Further removal of

temperature steps 890, 940 ◦C maintains the same age (868 ± 14 Ma)

but with improved MSWD (10)(40Ar/36Ar intercept = 303.5 ± 8).

These ages are statistically younger than U-Pb zircon ages for the

RIC (∼60 Ma) but are concordant at the 95 per cent confidence level

with 40Ar/39Ar hornblende cooling ages (867 and 875 Ma; Fig. 1)

from the surrounding RVGC (Bingen et al. 1998).

The Egersund dyke (Site 10; Sample E16D in Table 1; Fig. 2b)

yielded a near WMPA 40Ar/39Ar biotite age of 600 ± 5 Ma for

the seven last high temperature steps (80 per cent gas). This age

estimate is statistically younger than the U-Pb baddeleyite age for

the same dyke location (616 ± 3 Ma; Bingen et al. 1998) but overlaps

with a simple mean age (615 ± 36 Ma) for the same temperature

interval. The biotite age is to a large extent weighted on the last

two steps, and calculating an age over five intermediate to high

temperature steps yields a WMPA of 609 ± 10 (53 per cent gas)

that is statistically concordant with the U/Pb baddeleyite age. In the

subsequent discussion we refer to the Egersund dykes as 616 Ma

C© 2007 The Authors, GJI, 168, 935–948



Figure 3. Examples of thermal demagnetisation results from the Egersund dykes shown in orthogonal vector plots. Open (solid) symbols represent projections

onto the vertical (horizontal) planes, respectively.

but acknowledge remanence acquisition may have occurred between

616 and 600 Ma.

Palaeomagnetic data

The natural remanent magnetization (NRM) was measured on a

JR5A spinner magnetometer with a sensitivity of 10−5 A/m. A total

of 133 specimens from the 13 dyke sites, and an additional 38 speci-

mens from the host rock (mainly anorthosites and noritic dykes)

were demagnetised. Both alternating field (2G demagnetiser)

and thermal (MMTD1 furnace) demagnetisation were employed.

Although both methods yielded similar components, thermal

demagnetisation was preferred for the majority of the specimens,

mainly because alternating field demagnetisation gave rise to

spurious components attributed to Gyroremanent magnetisation

(GRM) at some sites (Stephenson 1981). Characteristic remanence

components were calculated using principal component analysis

(Kirschvink 1980).

Examples of thermal demagnetization behaviour of samples from

the Egersund dykes are shown in Fig. 3 and summarized in Table 2.

Most sites reveal steep downward pointing characteristic remanence

components (ChRC), with maximum blocking temperatures close to

580◦C, in accord with magnetite as the dominant magnetic mineral.

One site (site 4; Table 2) did not yield consistent directions, and was

excluded from further analysis. The remaining 12 sites display some

variability in remanence quality, with Fisher precision parameter kranging from 11 to 132.

Secondary components, with maximum unblocking temperatures

between 300 and 450 ◦C and steep positive inclinations, are also

present at most sites (Figs 3a and d). As may be seen from Fig. 4,

these low stability components yield a mean direction almost iden-

tical to the present day field. They probably constitute a young over-

print of viscous origin, and are not further elaborated here.

Of the 12 sites that yielded meaningful mean ChRC directions,

three (sites 3, 7 and 9) plot slightly outside the main cluster (Fig. 4).

C© 2007 The Authors, GJI, 168, 935–948



Table 2. Site mean directions and locations for the Egersund dykes. J , mean NRM intensity; N , number of demagnetised samples/sites;

n, number of samples/sites used to calculate means. Dec., mean declination; Inc, mean inclination; α95, 95 per cent confidence circle

of mean; k, Fisher precision parameter; UTM location: grid coordinates of individual sites using WGS 84 map datum; VGP, virtual

geomagnetic pole; dp/dm, semi-axes of 95 per cent confidence around the pole and Plat: corresponding palaeolatitude of Egersund area.

UTM locationSite J n (N) Dec. Inc α95 k

(A m−1) (◦) (◦) (◦) Zone x y

Egersund dykes1 0.260 8 (10) 67 76 14.8 15 32VLK 2845 7537

2 0.440 8 (8) 126 89 18.0 11 32VLK 2853 7542

3 0.197 14 (16) 146 32 11.1 14 32VLK 2486 8500

4 0.617 7 (7) (73) (−6) (38) (4) 32VLK 2437 8198

5 0.741 9 (11) 113 59 8.4 39 32VLL 2223 0510

7 0.257 5 (12) 259 74 10.9 50 32VLK 4428 6589

9 0.080 11 (14) 179 41 13.7 12 32VLK 3074 7392

10 1.238 7 (10) 144 78 6.2 132 32VLK 4009 7612

12 0.901 7 (9) 119 67 9.8 13 32VLK 2562 8058

13 0.417a 7 (9) 122 59 6.9 77 32VLK 3077 7424

14 0.033 7 (7) 87 83 11.7 28 32VLK 5447 6152

15 2.523a 6 (10) 133 54 20.6 11 32VLK 2434 8914

16 0.732 9 (10) 128 50 7.1 53 32VLK 3773 9910

aAfter removal of specimens with anomalous intensities attributed to lightning strikes

Mean Egersund dykes (excluding site 4)

12(13) 137 68 12.8 12VGP, 24.8N, 34.9E, dp = 17.9, dm = 21.4 (Plat = 50.7)Mean Egersund dykes (excluding site 3, 4, 7 and 9)

9 (13) 120 69 10.0 28VGP, 31.4 N, 44.1 E, dp = 14.5, dm = 17 (Plat = 52.8)

Figure 4. Site mean directions for the Egersund dykes. Characteristic remanence components (left) and low stability components (right) shown with α95

confidence circles for individual sites. Star in right hand diagram indicates the direction of the present day field. The three sites shown with stipled confidence

circles (3, 7 and 9) were omitted when computing the final mean and pole position (see text for discussion).

Sites 3 and 9 have shallower inclinations than the remaining sites,

and also show evidence of a more complex remanence structure

with several overlapping components (cf. Fig. 3d). Site 7 has a more

westerly declination than the main group. Mean directions and pole

positions were calculated both for all 12 sites, and after removal

of the three sites mentioned above. Data for both alternatives are

presented in Table 1. We note that removing the three outliers has

a limited effect on pole position and palaeolatitude, changing the

latter from 51◦ to 53◦. However, an improvement in statistical pre-

cision together with the complex nature of the remanence of the

shallow sites, leads us to prefer the second alternative, yielding a

palaeomagnetic pole at 31.4N, 44.1E (dp = 14.5; dm = 17.0), which

C© 2007 The Authors, GJI, 168, 935–948



Figure 5. Contact tests for sites 5 and 15. Dyke widths are 20 and 6 m, respectively. Stereonets show characteristic remanence directions for dykes (triangles),

baked zone in host rock (squares) and unbaked host rock (circles), respectively. Orthogonal vector plots give examples of thermal demagnetization of host rock

samples at moderate distances from dyke contacts, showing dual component remanences reflecting both dyke and host rock (RIC) characteristic remanence

directions. Inset at lower left shows enlargement of the final demagnetization steps for the site 15 specimen. Dyke width at the sites is 30 m for site.

compares favourably with the earlier studies of Storetvedt (1966)

and Poorter (1972) (Table 4).

In view of the questions that have been raised about the possibility

of remagnetization of the dyke system (Eneroth 2002; Eneroth &

Svenningsen 2004), contact tests were performed at three of the

sites; 5, 15 and the anomalously shallow site 9.

Results of the contact tests for sites 5 and 15 are presented in

Fig. 5. Site 5 is in the largest dyke, which has a width of approxi-

mately 20 m at the site location. As may be seen in Fig. 5a, the host

rock close to the dyke margins is completely overprinted with the

dyke direction, while specimens at a larger distance from the dyke

show both a partial dyke overprint and retention of a steep westerly

and upwards pointing direction in the highest blocking tempera-

ture range. This latter direction has been found by several authors

(Poorter 1972; Stearn & Piper 1984; Brown & McEnroe 2004) to

be the characteristic remanence direction in the Sweconorwegian

host rocks. A similar pattern for the 6 m wide dyke at site 15 is

illustrated in Fig. 5(b), with the specimens at 4.8 m distance from

the margin showing influence of both the dyke- and Sweconorwe-

gian directions, while the specimens at 12 m seem unaffected by

the dyke. Clearly, the results at both sites constitute strong, if not

conclusive evidence that remanence in the Egersund dykes is indeed

primary.

The contact test attempted at the anomalously shallow site 9,

shows a more complex relationship between remanence in the host

rock, baked zone and dyke (Fig. 6). The dyke and the contact mag-

netization clearly differ, whilst non-baked RIC samples show the

typical upward pointing inclinations with westerly declinations and

almost single component behaviour. We suspect that sites 3 and

9 might represent low-temperature hydrothermal remagnetizations,

probably of Late Ordovician origin when compared with the appar-

ent polar wander (APW) path for Baltica. Similar remagnetizations

C© 2007 The Authors, GJI, 168, 935–948



Figure 6. Contact test for site 9 (dyke width 4 m), showing mean remanence directions for the dyke, baked zone in host rock (0–2 m) and unbaked host rock

(2–20 m), respectively. Orthogonal vector plots show examples of thermal demagnetization behaviour for each group.

Table 3. Site mean directions for Sveconorwegian rocks of the RIC. See Table 2 for legend.

UTM locationSite Rock type J n (N) Dec. Inc α95 k

(A m−1) (◦) (◦) (◦) Zone x y

Sweconorwegian rocks6 Norite 0.295 9 (10) 028 −66 16 11 32VLK 2552 8100

8 Norite 0.834 6 (7) 273 −70 8.9 58 32VLK 5578 6307

11 Norite 0.902 7 (10) 240 −77 8.4 53 32VLK 3998 7392

Host 5 Gneiss 0.602 2 (2) 292 −66 (24.0) (106) 32VLL 2223 0510

Host 9 Anorthosite 0.184 7 (7) 292 −65 2.2 736 32VLK 3074 7392

Host 15 Anorthosite 0.600 2 (2) 222 −70 (15.1) (274) 32VLK 2434 8914

Mean Sweconorwegian rocks (excluding site 6)

5(6) 269 −72 11.0 49VGP: 45.9S, 238.4E, dp = 17.0, dm = 19.3 (Plat = 56.7)

(Caradoc?) are identified in SW Sweden (Scania) and attributed to

Baltica-Avalonia docking by Torsvik & Rehnstrøm (2003).

The three sampled noritic dykes, may be used together with

anorthosite samples from the three contact test sites (i.e. those taken

at sufficient distance from the dykes to avoid any thermal dyke in-

fluence) to provide a total of six sites in the RIC/RVGC (Table 3),

although the number of samples at two of these sites are limited.

Thermomagnetic curves for the norites are reversible with a single

Curie point of 580◦C indicating pure magnetite as the dominant

magnetic phase. The anorthosites show evidence of both magnetite

and hemoilmenite, with the bulk of the remanence being unblocked

between 550 and 580◦C, but with directional stability being retained

up to 680◦C in some cases. After the removal of lower stability sec-

ondary components (Fig. 7a), both rock types reveal a consistent

high quality ChRC with steeply dipping reverse directions (Fig. 7).

Fig. 8 and Table 3 summarize the statistics from the six RIC sites.

Removal of one deviating site (Fig. 7) leads to a pole position of

45.9S, 238.4E, (dp = 17.0, dm = 19.3) and a local palaeolatitude of

57◦. This is in broad agreement with the findings of previous studies

(Poorter 1972; Stearn & Piper 1984; Brown & McEnroe 2004) as

shown in Table 4 and Fig. 11. Thus, the main problem in the RIC,

seems to be the timing rather than the direction of remanence, which

will be briefly discussed in the later sections.

Rock magnetic properties

Various rock magnetic methods were employed in an attempt to de-

termine the nature and stability of the remanence carriers, including

thermomagnetic curves, coercivity spectrum analysis and optical

microscopy. We also measured the anisotropy of magnetic suscep-

tibility (AMS), in part to test for potential remanence deflections

(Cogne 1987). Mean anisotropy values for all Egersund dyke sites

are below 5 per cent, suggesting that remanence deflections are not

significant. However, anisotropy in the anorthosites and norites of

the RIC is above 50 per cent in some cases, indicating that remanence

deflection might be a significant source of error both in this and all

previous palaeomagnetic studies of the Sweconorwegian rocks. The

AMS results are not further elaborated here.

Reflected-light microscopy (Fig. 9) reveals relatively large titano-

magnetite as the dominant magnetic mineral phase in all the dykes,

C© 2007 The Authors, GJI, 168, 935–948



Figure 7. Examples of thermal demagnetization behaviour of RIC (a) and

RVGC (b) specimens. Conventions as in Fig. 3.

with ilmenite lamellae attesting to high temperature oxidation and

a relatively slow initial cooling rate. Quite severe secondary low

temperature oxidation is evident at several sites (Fig. 9b and c).

However, isothermal remanent magnetisation (IRM) (Fig. 10a) ver-

Table 4. Selected pole postions for Baltica 1100–550 Ma.

Formation Location Age Pole dp/dm or A95 Palaeo-latitude Reference

Winter Coast sediments 66N, 40 E 555 ± 3 25N, 312E 2/4 24 Popov et al. (2002)

Egersund dykes 58 N, 6 E 616 ± 3 28N, 52E 15/18 46 Storetvedt (1966)

Egersund dykes 58 N, 6 E 616 ± 3 22N, 51E 16/21 42 Poorter (1972)

Egersund dykes 58 N, 6 E 616 ± 3 31N, 44E 15/17 53 This study

Neoproterozoic combined 70N, 31E ≈ 750 28N, 197Eb 8 8 Meert & Torsvik (2003)

Hunnedalen dykes 59N, 7E 848 ± 27 41S, 222E 10/11 62 Walderhaug et al. (1999)

Egersund anorthosite 58N, 6E 869 ± 14a 42S, 200E 9 71 Brown & McEnroe (2004)

RIC/RVGC 58N, 6E 869 ± 14a 46S, 238E 17/19 57 This study

RIC/RVGC combined 58N, 6E 869 ± 14a 43S, 213Eb 5 68 Meert & Torsvik (2003)

Bamble intrusions mean 59N,10E ≈1060 3N, 217 Eb 15 24 Meert & Torsvik (2003)

aAge based on 40Ar/39Ar dates on biotite; not U-Pb age (930 Ma) used by original authors (see text for discussion).bPole polarity inverted.

N

6

Figure 8. Site mean directions for RIC and RVGC. Conventions as in Fig. 4.

sus field curves reach saturation below 200 mT, showing no evidence

of high coercivity phases such as hematite.

Thermomagnetic curves, (Figs 10b, c and d) were obtained in

air on a horizontal translation balance with a heating/cooling cy-

cle of 100 min in a field of 0.7 Tesla. Although the detailed be-

haviour varies between sites, they all show Curie points close to

580◦C, suggesting that the magnetic phase has a composition close

to pure magnetite. Hysteresis measurements, performed on a Mol-

spin vibrating sample magnetometer, yield mean J rs/J s ratios of

0.16, suggesting PSD grains of moderate stability. (Dunlop 1986;

Dunlop & Ozdemir 1997), although interpretation of this ratio in

terms of domain state is somewhat ambiguous when mixtures of

different grain sizes and varying titanomagnetite compositions are

involved (Tauxe et al. 1996; Dunlop 2002).

I N T E R P R E TAT I O N

Egersund dykes

Fig. 4 and Table 2 summarize site mean directions from the Egersund

dykes. After excluding sites 3, 7 and 9 as discussed above, the

Egersund dykes average to 120◦ (declination) and +69◦ (inclina-

tion). This yields a palaeolatitude for Southwest Baltica of 53◦

C© 2007 The Authors, GJI, 168, 935–948



Figure 9. Reflected light micrographs of titanomagnetite grains from the

Egersund dykes. Length of scale bars = 100 μm.

(+16◦/−13◦) at around 616 Ma (the U-Pb baddeleyite age). The

Egersund pole (Table 2) overlaps with earlier studies of the same

dykes (Fig. 11) but also plots in the vicinity of well-established

Early Ordovician poles from Baltica. However, the Egersund dykes

are undeformed, U/Pb and Ar–Ar ages differs only marginally, and

two positive contact tests and an opposite polarity compared to the

host rocks convincingly point to a primary origin. Hence, the simi-

larity between Ediacaran poles and Early Ordovician poles (Fig. 11)

is in our view coincidental, and not due to remagnetization.

Egersund host-rocks (RIC)

The Egersund dykes are of opposite polarity to the sites in the host

rocks (mean inclination = −72◦; palaeolatitude = 57◦). This in it-

self suggests a positive regional stability test for the Egersund dykes.

We combined four RIC sites with site 5 from RVGC (only two sam-

ples) since they are identical (Fig. 8). The dated noritic dyke (869 ±14 Ma) is somewhat anomalous in declination, but we have no rea-

sons to assume that this dyke is different in age from the other dykes.

The RIC is dated to between 929 and 932 Ma (U-Pb zircon) but the

869 Ma 40Ar/39Ar biotite age (this study) and 40Ar/39Ar hornblende

ages of 875 ± 20 and 867 ± 2 Ma from RVGC are in our opinion

more realistic estimates for remanence acquisition since they date

uplift/cooling between 300 and 525◦C.

The RIC/RVGC palaeomagnetic directions are also similar to the

848 ± 27 Ma Hunnedalen dykes (Fig. 11). These data are also of

the same magnetic polarity, leading Walderhaug et al. (1999) to

the conclusion that the RIC/RVGC record a younger regional uplift

event which we date to 869 Ma, and remanences in the RIC should

not be associated with the existing U-Pb ages (c. 932 Ma).

N E O P RO T E RO Z O I C - E D I A C A R A N

P O L E S F RO M B A LT I C A

The Precambrian palaeogeographic position for Baltica and its re-

lation to other continents is patchy and poorly understood. For the

times when Baltica is considered to be part of the Rodinia super-

continent there are three undisputed data-sets:

(i) Sveconorwegian poles (∼970–1100 Ma) suggesting low-to-

subtropical latitudes for Baltica.

(ii) The Egersund anorthosites (interpreted here as 870 Ma up-

lift magnetizations as described in the previous section) and the

Hunnedalen dykes (850 Ma) in SW Norway indicating very high

latitudes.

(iii) 700–800 Ma sedimentary poles (not well dated) from North-

ern Norway/Russia that demonstrate equatorial latitudes.

During and after Rodinia breakup, we have demonstrated that

Baltica had returned to intermediate-to-high latitudes (50–75◦S)

by ∼616 Ma (Fig. 12). In the 1100–616 Ma range, severely in-

terpolated apparent polar wander (APW) rates do not exceed

10 cm/yr., latitudinal velocities are always less than 8 cm yr−1 (ca.

4 cm yr−1 on average), thus indicating ‘normal’ plate tectonic

speeds. For the remaining part of the Ediacaran and the Lower

Cambrian, palaeomagnetic poles from Baltica are profoundly con-

fusing, and accepting all published results would lead to an unten-

able APW path (Meert et al. 2006). Similarly confusing results on

other continents have led to proposals or True Polar Wander during

this time period (Kirschvink et al. 1997), but this interpretation is

contentious (Meert 1999). Although some component of true po-

lar wander cannot be ruled out, we prefer to explain the disparate

Baltica data more conventionally in terms of remagnetization, poor

age constraints and tectonic coherency affecting individual results.

In this respect, the most robust result from this period appears to

be the 555 Ma Winter Coast sediment pole of Popov et al. (2002)

shown in Fig. 11. A simple post-600 Ma continuation of the Baltica

APW path involving only the Egersund, Winter Coast, and (non-

contentious) Ordovician poles (Fig. 11) would indicate a ca. 30◦

Northward shift of Baltica (from the position depicted in Fig. 12)

by the Late Ediacaran, followed by a return to higher latitudes by

the Early Ordovician. We stress, however that emphasizing other

available poles leads to different conclusions (see Meert et al. 2006,

C© 2007 The Authors, GJI, 168, 935–948



0

5000

10000

15000

20000

25000

0 100 200 300 400 500 600 700 800

E84SITE 10

0 100 200 300 400 500 600 700

J

Temp (°C)

0

1000

2000

3000

4000

5000

6000

0 100 200 300 400 500 600 700 800

E29SITE 7

0 100 200 300 400 500 600 700

Temp (°C)

Temp (°C)

J

a) b)

c) d)

-50 100 200 300

Field (mT)

JIRM

Figure 10. Representative examples of isothermal remanence versus field curves (a) and thermomagnetic curves (b–d) from the Egersund dykes.

Figure 11. Selected pole positions for Baltica. Early- and Mid-Ordovician poles listed in Torsvik & Rehnstrøm (2003). All other poles are specified in Table 4.

Galls projection.

for discussion), and new robust data from the Late Ediacaran and

Early Cambrian are clearly desirable.

Ediacaran palaeomagnetic poles from Laurentia are also chaotic,

thus making it hard to make any conclusive statements about the rel-

ative position of Baltica versus Laurentia during this period. Palaeo-

magnetic results from the only rocks that are contemporaneous with

the Egersund dykes, i.e. the 615 ± 2 Ma (U-Pb baddeleyite) Long

Range dykes (Kamo et al. 1989) in Labrador (Canada) are highly

ambiguous yielding four different results (pick high or low latitude

position to your preference). However, if the Iapetus Ocean opened

between Baltica and Laurentia around this time, Laurentia must be

close to the Baltic margin. The Egersund result, therefore, suggests

that the higher latitude options for Laurentia are more plausible.

Younger Ediacaran poles from Laurentia (as for Baltica) are equally

C© 2007 The Authors, GJI, 168, 935–948



Figure 12. Reconstruction of Baltica at 616 Ma based on the result in this study. Stars indicate locations of the <620 Ma Moelv Tillite (Bingen et al. 2005)

and the Mortensnes glacial deposits (North Norway). A tentative location of the Timan terranes is shown. These terranes coalesced with Baltica at around

550 Ma (see text). FZ, Franz Josef Land; NZ, Novaya Zemlya.

ambiguous and cannot yet be used for detailed palaeogeographic

reconstructions towards the dawn of the Cambrian.

B A LT I C A A N D VA R A N G E R I A N

G L A C I AT I O N S

Neoproterozoic and Vendian glacial deposits are recorded on many

continents, some evidently deposited at low latitudes and thereby

suggesting that Earth was affected by global glaciation events

(e.g. Hoffman & Schrag 2002). In the Varangerfjord area of northern

Norway, two stratigraphically separate glacial units, the Smalfjord

(lower) and Mortensnes (upper) formations, occur (Edwards 1984).

Halverson et al. (2005) argue that the Smalfjord formation repre-

sents the worldwide ‘Marinoan’ (ca. 635 Ma) glaciation based on

the presence of characteristic δ13C anomalies and a cap carbonate.

On the basis of sequence stratigraphy, the Mortensnes formation is

presumed to correlate with the Moelv tillite in Southeast Norway

(Fig. 12). These two latter deposits have been suggested to represent

the ‘Gaskiers’ (580 Ma) glaciations (Bingen et al. 2005; Halverson

et al. 2005), but lack distinctive features such as a cap carbonate.

The Neoproterozoic glacial formations in Norway are in general

poorly dated. Recently however, U/Pb dating of detrital zircons has

constrained the deposition of the Moelv Tillite to less than 620 ±14 Ma (Bingen et al. 2005).

Palaeomagnetic data from the Egersund dykes clearly demon-

strate that Baltica was located at relatively high latitudes close to

the time of the Varangerian glaciation, stretching from ca. 50◦S

(present Caledonian margin) to ca. 75◦S (present Southern Urals)

(Fig. 12). These data certainly do not rule out the postulate of a

global glaciation, but no global ‘Snowball Earth’ glaciation is re-

quired to account for the glacial deposits on Baltica (see also Bingen

et al. 2005).

Williams (1975, 1993) has proposed an alternative explanation

for Neoproterozoic low latitude glaciations. If the Earth’s orbital

obliquity were substanitially higher (>54◦) than the present value

of 23.5◦, the Earths climatic zonation would reverse, and glaciation

would preferentially occur at equatorial latitudes. This model does

not provide a ready explanation for the Varanger and Moelv glacial

deposits, since glaciations would be largely constrained to latitudes

below 40◦ (Williams 1993).

We will emphasize that the Ediacaran definition of Baltica was

very different from its Palaeozoic boundaries. During the Ediacaran

(Fig. 12), today’s northern part of the northwestern margin of Baltica

changed from an extensional tectonic regime to an active margin.

These changes, termed the Timanide (or Timanian) Orogeny, repre-

sent a period of active accretion in which various microcontinental

blocks in the Timan-Pechora, northern Ural and Novaya Zemlya ar-

eas were united with Baltica to form a much expanded terrane area

at ca. 555 Ma (Gee et al. 2000; Rehnstrom et al. 2002; Roberts &

C© 2007 The Authors, GJI, 168, 935–948



Siedlecka 2002; Roberts & Olovyanishnikov 2004; Cocks & Torsvik

2005). Vendian magmatism, metamorphism and tectonic activity in

the Timan-Pechora-Rybachi-Varanger region was broadly contem-

poraneous with the Varangerian glacial event. In Fig. 12, we have

tentatively shown the Timan Terranes as a single unit (for reasons of

simplicity) at some distance from Baltica, indicating that they may

have been located at lower latitudes than Baltica before their final

Late Ediacaran convergence (ca. 555 Ma).

C O N C L U S I O N S

Palaeomagnetic data from the Egersund dykes yield moderately

steep magnetisations that witness an intermediate-to-high latitude

position of Baltica. Contact tests provide strong evidence that the

magnetization is primary. One dyke yields a 40Ar/39Ar biotite age of

600 ± 10 Ma, only marginally younger than the previous U-Pb bad-

deleyite age (616 ± 3 Ma). We therefore, conclude that remanence

acquisition took place between 600 and 616 Ma, and that Baltica

was located at latitudes higher than 50◦ during this time.

The magnetic signature of the RIC and RVGC host rocks differs

from the Egersund dykes, the magnetic polarity is opposite, and

palaeomagnetic results are broadly similar to earlier published re-

sults. A noritic dyke from the RIC yields a 40Ar/39Ar biotite age

of 869 ± 14 Ma and approximately 60 million years younger than

U-Pb zircon ages from RIC. 869 Ma is considered as a cooling age

and is concordant with 40Ar/39Ar hornblende ages from the RVGC.

Comparable magnetizations and 40Ar/39Ar ages from RIC-RVGC

clearly suggest that the magnetizations are not primary but represent

uplift/cooling ages. This is also sustained by similarity in magnetic

signature with the ca. 850 Ma Hunnedalen dykes (Walderhaug et al.1999). We therefore, argue that the remanence recorded in RIC-

RVGC is related to approximately 870 Ma uplift/cooling through

300–525 ◦C.

A C K N O W L E D G M E N T S

Field work in the Egersund area was carried out with funding from

NFR, VISTA and NGU. Constructive and thorough reviews by Tim

Raub and Joe Meert greatly improved the manuscript. Bernard Bin-

gen is thanked for providing the basis for Fig. 1, and Elizabeth Eide

for providing assistance with the Argon dating.

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